Variable ultrasonic delay line



VARIABLE ULTRASONIC DELAY LINE Filed Dec. 20, 1961 INVENTOR. HOLDEN P.WRIGHT United States Patent 3,146,412 VARIABLE ULTRASONIC DELAY LINEHolden P. Wright, Bolton, Conn., assignor to Laboratory for Electronics,Inc Boston, Mass., a corporation of Delaware Filed Dec. 20, 1961, Ser.No. 160,773 1 Claim. (Cl. 33330) This invention pertains generally tosolid ultrasonic delay lines and particularly to improved lines of suchtype in which the delay time may be varied.

In known variable delay lines, except the type described in myco-pending application Serial No. 139,454, filed September 20, 1961, andassigned to the same assignee as the present invention, delay time isvaried by varying the length of the path of acoustic energy in anhomogeneous acoustic transmitting medium. In my cited co-pendingapplication, on the other hand, the length of the path of acousticenergy is substantially constant, variations in delay time beingaccomplished by directing an acoustic beam through a non-homogeneousacoustic transmitting medium so that the time taken for such a beam totravel from a transmitting transducer to a receiving transducer may bevaried.

While the structure shown in my cited co-pending application has provento be useful, the fact that both the transmitting acoustic transducerand the receiving acoustic transducer must be moved simultaneously hasgiven rise to some difiiculty. It is evident that mechanical tolerancesmust be reduced to the lowest practicable value if the full advantagesof the cited invention are to be attained. It is also evident that, evenif mechanical tolerances are kept within close limits, it may benecessary to move the two transducers relative to each other to attainbest results.

Therefore, it is an object of this invention to provide a variable soliddelay line requiring only one movable acoustic transducer to cover arange of delay times.

Another object is to provide an improved variable solid delay line inwhich the path of acoustic energy is kept sub stantially constantthroughout a range of delay times.

Still another object of the invention is to provide a solid acousticdelay line in which the foregoing objects are attained in a mechanicallysimple and rugged device.

These and other objects of the invention are attained generally byproviding an acoustic transmitting medium consisting of two wedges ofdifferent kinds of acoustic transmitting material joined together toform a unitary block, a transmitting transducer aifixed to one of thewedges to flood the interface between the two wedges with acousticenergy so that such energy is there refracted to a movable receivingtransducer. For a more complete understanding of the invention referenceis now made to the drawings in which:

The single figure is a perspective view, partially cutaway and greatlysimplified, of a preferred embodiment of the invention.

Referring now to the figure, it may be seen that a delay line accordingto the invention comprises an acoustic transmitting assembly mountedwithin a case 11. The acoustic transmitting assembly 10 consists of afirst wedge 13 of material, as fused quartz, bonded to a second wedge 15of a different material, as glass, at an interface 17. Preferably theinterface 17 is formed by grinding the opposing surfaces of the wedges13, 15 and cementing the two surfaces together with a bonding material,as phenyl benzoate. A mounting bracket 19 screwed to an end wall of thecase 11 serves to clamp the acoustic transmitting assembly in place, asillustrated. A relatively long acoustic transmitting transducer 21, as apiezo-electric crystal, is cemented, again as by a film of phenylbenzoate to a surface of the first wedge 13. The transducer 21 is ener-3,146,412 Patented Aug. 25, 1964 gized from any convenient known source(not shown) through lead wires 23. An acoustic receiving transducer 25,as a piezo-electric crystal, is cemented, again as by a film of phenylbenzoate, to an acoustic coupler 27, as glass, which is pressed againstthe second wedge 15 by the action of a spring 29 as shown. A lead screw31 is connected on one end to the acoustic coupler 27 and led through athreaded bushing 33 in an end wall of the case 11. A hand wheel 35aflixed to the outer end of the leadscrew 31 completes the assembly.

In operation, the acoustic transmitting transducer 21 is energized withthe signal to be delayed. Acoustic energy is then projected through thefirst wedge 13 to impinge all along the interface 17 The energy is thererefracted and directed through the second wedge 15. A portion of therefracted energy is intercepted by the acoustic coupler 27 and directedto the receiving acoustic transducer 25, there being converted into anelectric signal and led to any appropriate utilization circuit (notshown) through the unnumbered leads projecting from the receivingacoustic transducer 25.

Variations in delay time are obtained by rotating the hand wheel 35 toturn the lead screw 31 in the threaded bushing 33, thereby causing theacoustic coupler 27 to move along the surface of the second wedge 15. Asa result of such movement of the acoustic coupler 27, different portionsof the energy propagated by the acoustic transmitting transducer 21 areintercepted by the acoustic coupler 27. Consequently, the receivingacoustic transducer 25 is energized by energy which has been delayed forvarying times.

It will be apparent that the shape and material of the various elementsmaking up the acoustic path may be Varied within wide limits, dependingon the characteristics of the chosen material and the desired range ofdelay times. It is essential only that the velocity with which acousticenergy is passed through the first and second wedges 13, 15 differs andthat the Wedges be so shaped that some of the acoustic energy isrefracted rather than reflected at the interface 17. In a practicalcase, for example a range of delay between and microseconds may beattained in a device in which the first wedge 13 is fused quartz and thesecond wedge 15 and the acoustic coupler 27 are glass and the wedges arecut approximately as shown in the drawing. To be specific (referring tothe figure) to attain such a range of delays: (the velocity ratiobetween the two materials being approximately 3:2)

fiE=12%" It will be recognized, however, that variations in the justgiven dimensions may be required to compensate for variations in thepropagation constants of the materials used.

It will be apparent that changes other than those suggested above willbe evident to those having skill in the art. For example, the functionsof the receiving and transmitting transducers may be exchanged withoutaffecting the invention. In addition, known absorbers and lubricants maybe used to minimize the effect of spurious reflections and to render thesliding contact between the wedge 15 and the acoustic coupler 27 moreefiicient. It is felt, therefore, that the invention should not berestricted to the specific embodiment illustrated but rather should belimited only by the spirit and scope of the appended claim.

(e) an acoustic coupler of glass slidably mounted on a second surface ofthe wedge of glass, the surface of the acoustic coupler removed from thewedge of glass being normal to the path of acoustic energy in the wedgeof glass;

(f) a receiving acoustic transducer affixed to the surface of theacoustic coupler removed from the wedge of glass to receive acousticenergy intercepted by the acoustic coupler; and,

(g) means for moving the acoustic coupler and the receiving acoustictransducer along the second surface of the wedge of glass.

References Cited in the file of this patent UNITED STATES PATENTS2,602,101 Mesh July 1, 1952 2,672,590 McSkimin Mar. 16, 1954 2,703,867Arenberg Mar. 8, 1955 2,781,494 Geoghegan Feb. 12, 1957 2,851,884Arenberg Sept. 16, 1958 2,996,687 Wright Aug. 15, 1961

